1. Field of the Invention
The present invention relates generally to an apparatus and method for using an Automatic Repeat reQuest (ARQ) in a Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for efficiently using an ARQ retransmission timer of a Media Access Control (MAC) layer.
2. Description of the Related Art
Research is being actively conducted to provide services with various QoS (Quality of Service) at a transmission speed of about 100 Mbps in the 4th Generation (4G) communication system, which is a next-generation communication system. In particular, the research has evolved to guarantee the mobility of and the QoS of the existing Broadband Wireless Access (BWA) communication systems, such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system. A representative example of the BWA communication systems is the IEEE (Institute of Electrical and Electronics Engineers) 802.16 communication system.
For a physical channel, the BWA communication systems, such as the IEEE 802.16 communication system, have employed the Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access scheme (hereinafter, the “OFDM/OFDMA” scheme).
During wireless data communication, an error may occur in particular data, depending on the channel state of a radio resource section. Techniques of controlling and restoring data caused by such an error may be largely categorized into the ARQ (Automatic Repeat reQuest) technique and the FEC (Forward Error Check) technique. The ARQ technique is used to request retransmission of data that is lost during transmission to a receiver, and the FEC technique is used to correct an error in data that occurs during transmission to a receiver.
In particular, in the ARQ technique, a result of error detection for packets received by a receiver, e.g., a CRC (Cyclic Redundancy Check) result, must be fed back to a sender. First, when the sender initially transmits the packets, the receiver receives and decodes the packets. In this case, if an error does not occur, the receiver transmits an ACK signal to the sender. If an error occurs, the receiver transmits a NACK signal to the sender. Then, the sender retransmits the transmitted packets or transmits new packets, depending on whether the receiver transmits the ACK signal or the NACK signal.
The BWA communication system employs two retransmission methods for reliable data transmission. One of the methods is the MAC (Media Access Control) ARQ method employed by the MAC layer, and the other method is the HARQ (Hybrid ARQ) method employed by a physical (PHY) layer. In general, the MAC ARQ method requires 50 to 80 ms as to perform data retransmission once, but the HARQ method is capable of rapidly performing data retransmission within 10 to 30 ms. That is, in the HARQ method, it is possible to perform data retransmission several times for a short period of time by minimizing the amount of time for the sender to spend waiting for an ACK signal, thereby increasing the rate of data transmission.
However, an ACK/NACK signal when using the HARQ method is less reliable than when using the MAC ARQ method, thus lowering the reliability of data transmission. Also, the HARQ method does not guarantee the sequence of transmitted data, since a retransmission window is not available. In contrast, the MAC ARQ method guarantees reliable data transmission and the sequence of transmitted data. For this reason, in general, the HARQ method and the MAC ARQ method are used together.
FIG. 1 illustrates the format of a Packet Data Unit (PDU) of a MAC layer in a conventional BWA communication system.
As illustrated in FIG. 1, the PDU 107 of the MAC layer, which is transmitted in a radio section, has a format in which a generic MAC header 101 is added before a payload field 103, and an optional CRC (Cyclic Redundancy Check) field 105 is added after the payload field 103. Thus, data is retransmitted in units of PDUs during an ARQ mode.
FIG. 2 is a state transition diagram of an ARQ of a MAC layer in a conventional BWA communication system.
As illustrated in FIG. 2, the states of the ARQ include a “not sent” state 200, an “outstanding” state 202, a “wait for retransmission” state 204, a “discarded” state 206, and a “done” state 208.
Referring to FIG. 2, first, when a packet (a MAC PDU) is transmitted in the “not sent” state 200, the MAC layer transmits to the “outstanding” state 202 in order to drive an ARQ retransmission timer. If a NACK message is received from a receiver or a predetermined retransmission timer ends in the “outstanding” state 202, the MAC layer transmits to the “wait for retransmission” state 204. The MAC layer that transmits to the “wait for retransmission” state 204 retransmits the packet, and transmits to the “outstanding” state 202 again.
The MAC layer transmits to the “done” state 208 when an ACK message is received from the receiver in the “wait for retransmission” state 204, and transmits to the “discarded” state 206 when data retransmission continuously fails and a predetermined lifetime ARQ_BLOCK_LIFETIME expires. When the ACK message is received from the receiver in the “outstanding” state 202, the MAC layer transmits to the “done” state 208. Also, when the lifetime ARQ_BLOCK_LIFETIME expires in the “outstanding” state 202, the MAC layer transmits to the “discarded” state 206 so as to discard the packet. When the ACK message is received from the receiver in the “discarded” state 206, the MAC layer transmits to the “done” state 208.
As described above, the MAC layer of a sender transmits an ARQ block, and then waits for an ACK message to be fed back from the receiver. In this case, when the ACK message is normally received, control of data retransmission for the ARQ block is discontinued, but the ARQ block is retransmitted when the retransmission timer expires before the ACK message is received. Also, when data retransmission fails continuously and the lifetime of the ARQ block expires, the ARQ block is discarded.
In general, a period of time that the MAC layer of the sender stands by so as to receive an ACK message after transmitting data, is referred to as “the ARQ retransmission timer”. The ARQ retransmission timer is used to determine an instance of time when data retransmission will be performed when a transmission error occurs, and thus significantly influences the ARQ retransmission performance. The ARQ retransmission timer must be set to a minimum value within a range that guarantees a sufficient time for the receiver to transmit an ARQ response. In detail, the ARQ retransmission timer is determined in consideration of a delay time of transmission of data from the sender to the receiver, a length of time that the receiver spends receiving the data and preparing an ARQ response therefor, and a delay time of transmission of the ARQ response from the receiver to the sender. Since a delay time of transmission is very short when using only the ARQ of the MAC layer, the retransmission timer is determined by the length of time that the receiver spends preparing a response. In this case, the retransmission timer is approximately several tens msec. However, when the ARQ is used together with a HARQ, an additional transmission delay occurs due to retransmission of the HARQ, and thus, the ARQ retransmission timer of the MAC layer must be determined in consideration of a maximum delay time of retransmission of the HARQ. For example, when a maximum number of times that the HARQ can be transmitted is 3, a delay time of transmission due to retransmission of the HARQ is greater than or equal to 100 msec. Therefore, the ARQ retransmission timer of the MAC layer must be set to be greater than or equal to 100 msec.
As described above, when the MAC ARQ method and the HARQ method are used together, the ARQ retransmission timer of the MAC layer must be set in consideration of a delay in retransmission of the HARQ of a lower physical layer. If the ARQ retransmission timer is set to be significantly long in consideration of retransmission of the HARQ, retransmission delay is increased greatly. In contrast, when the ARQ retransmission timer is set to be short, the MAC layer may perform unnecessary retransmission, which will now be described in detail.
FIG. 3 illustrates a conventional case where data retransmission is delayed due to an ARQ retransmission timer of a MAC layer.
Referring to FIG. 3, a MAC PDU generated by the MAC layer of a sender is transmitted to a receiver via a physical layer PHY. The physical layer PHY of the receiver determines whether an error has occurred in received data HARQ Burst, and transmits a negative response HARQ-NACK to the sender when it is determined that an error has occurred. However, when an error occurs in the HARQ-NACK and a positive response HARQ-ACK is transmitted to the sender, the physical layer PHY of the sender determines that the HARQ is normally transmitted and thus does not retransmit the data HARQ Burst. In this case, the MAC layer of the receiver does not receive the MAC PDU and thus cannot transmit an ACK signal. Accordingly, the MAC layer of the sender waits until an ARQ retransmission timer expires, and retransmits the MAC PDU. As a result, retransmission of the ARQ of the MAC layer is significantly delayed since the retransmission timer is set to be long in consideration of retransmission of the HARQ. In this case, the average rate of transmission is degraded.
FIG. 4 illustrates a conventional case where data is unnecessarily retransmitted due to an ARQ retransmission timer of a MAC layer.
Referring to FIG. 4, a MAC PDU generated by the MAC layer of a sender is transmitted to a receiver via a physical layer PHY. The physical layer PHY of the receiver determines whether an error has occurred in received data HARQ Burst, and transmits a negative response HARQ-NACK to the sender when it is determined that an error has occurred. The physical layer PHY of the sender, which receives the negative response HARQ-NACK, retransmits the data HARQ Burst to the receiver. If the receiver normally receives the data HARQ Burst after the data HARQ Burst is retransmitted several times, the physical layer PHY of the receiver transmits a positive response HARQ-ACK to the sender, and the received MAC PDU to an upper MAC layer. Then, the MAC layer of the receiver determines whether an error has occurred in the received MAC PDU and transmits a positive response ARQ-ACK to the MAC layer of the sender when it is determined that the MAC PDU is normally received.
As described above, when transmission of the MAC PDU is delayed due to continuous data retransmission of the physical layer, the ARQ response from the MAC layer is also delayed. In this case, if the ARQ retransmission timer of the sender is not sufficiently long, the ARQ retransmission timer expires before the transmitter receives the ARQ response from the receiver, and therefore, the MAC layer of the sender unnecessarily retransmits the MAC PDU to the receiver. That is, although the MAC PDU is normally transmitted, the sender mistakenly determines that a transmission error has occurred and thus retransmits the MAC PDU unnecessarily, since the ARQ retransmission timer is short.